A government watchdog group is asking lawmakers to disclose special perks they receive from Delta Airlines as Congress struggles to approve long-term federal funding for aviation.

The Public Campaign Action Fund, a Washington, D.C.-based organization that advocates for public financing of elections, sent letters Tuesday to 20 lawmakers, including the three most senior Republicans in Congress, demanding that they reveal any “corporate benefits” they have received from the airline, including access to its “VIP hotline” or “special elite status.”

Lawmakers don’t seem likely to reach a compromise on reauthorization of the Federal Aviation Administration this year even after a gap in federal funding led to a two-week shutdown this summer. The stalemate is largely a result of a dispute over a 2009 ruling by the National Mediation Board that would make it easier for Delta employees to unionize.

Delta and its Republican allies are bent on using the funding bill to overturn the decision, which held that the decision to unionize an airline should be based on a majority of those who vote as opposed to those who are simply eligible to vote. The AFL-CIO and other unions such as the Association of Flight Attendants — which lost a representation election at Delta in 2006 — argued the old rule was unfair because it equated voter apathy with a “no” vote.

Delta, which is largely not unionized, has lobbied the issue intensively, spending $140,000 on contacts with Congress this year, according to federal records.

“Delta has a huge stake in this debate and has been the only airline lobbying on the anti-union National Mediation Board provision and in the process has poured campaign contributions into members’ campaign coffers,” wrote David Donnelly, PCAF’s national campaigns director. “We believe your constituents have a right to know.”

Roll Call reported in October that Delta has a Congressional call desk and allows Members to double-book flights. It is one of several service-oriented companies that operate in the Washington area that provide preferential treatment to Members of Congress and administration officials.

The letters cite Roll Call’s article and another report by the Atlanta Journal-Constitution alleging that Delta has given upgrades to local and state officials in Georgia, where the company is based.

So far this year, Delta’s political action committee has contributed more than $133,000 to lawmakers, including $5,000 to House Majority Leader Eric Cantor (R), who lives in Virginia, according to the Center for Responsive Politics.

“For Eric Cantor, he’s not flying back and forth to Washington, but he is flying all over the country to give speeches and raise money,” Donnelly said. “The question is: Is he getting that elite status not to support his work as a lawmaker, but to support his fundraising?”

Delta spokesman Trebor Banstetter said the contributions and perks are entirely legal and permissible by Congressional ethics standards.

“Delta follows guidelines established by Congress regarding travel and complies with Congressional ethics rules,” he told Roll Call. “Federal officials must earn any elite status with the airline.”

Earlier this week, Senate Commerce Chairman Jay Rockefeller (D-W.Va.) blamed the airline industry for a four-year delay in passing a funding bill for the FAA, citing the fight over the labor provision.

“I do not understand how the fixation of one airline can be seen as paramount in such that the House would shut down the FAA to get its way,” he said Monday in a speech.

House Transportation and Infrastructure Chairman Rep. John L. Mica (R-Fla.) has acknowledged that the labor obstacle is now in the hands of leadership of both parties.

The latest extension expires Feb. 1, and the labor hang-up is frustrating other airlines that want to see a fully funded four-year bill to provide certainty for capital projects like the so-called NextGen air traffic control upgrade.

On Tuesday, the Washington, D.C.-based Communications Workers of America, which is affiliated with the flight attendants union, went on the attack, calling Delta the “airline of the one-percent” for its efforts to lobby lawmakers.

The letter went to Cantor, Speaker John Boehner (R-Ohio) and 14 other House Members. Four Republican Senators also received the disclosure demand — Saxby Chambliss and Johnny Isakson of Georgia, Orrin Hatch of Utah and Minority Leader Mitch McConnell of Kentucky.